1. Field of the Invention
This invention relates to stable performance of molten carbonate fuel cells using sulfur containing fuels. Utilization of copper or copper alloy anodes in molten carbonate fuel cells increases tolerance to H.sub.2 S containing fuels, as compared to nickel electrodes, thereby providing stable fuel cell operation using directly fuel obtained from gasification of naturally occurring organic carbonaceous materials, such as coal.
2. Description of the Prior Art
The use of molten carbonate fuel cells is well known for the conversion of chemical energy directly into electrical energy by a galvanic oxidation process. Molten carbonate fuel cells generally comprise two electrodes with their current collectors, a cathode and an anode, an electrolyte tile containing alkali metal carbonates making contact with both of the electrodes, and a cell housing to physically retain the cell components. Under fuel cell operating conditions, generally about 500.degree. to about 700.degree. C., the entire electrolyte tile, the carbonate and the inert support material, forms a paste and thus the electrolyte diaphragms of this type are known as "paste electrolytes". The electrolyte is in direct contact with the electrodes where the three-phase, gas-electrolyte-electrode, reactions take place. Further details of construction and operation of high temperature carbonate fuel cells is set forth in U.S. Pat. Nos. 4,009,321 and 4,247,604 and the references referred to therein, all incorporated herein by reference.
One desirably used fuel for molten carbonate fuel cells is a mixture of gases comprising principally hydrogen, carbon dioxide, and carbon monoxide as obtained by gasification of naturally occurring carbonaceous material such as coal, shale or peat, as well known in the art. Gas mixtures obtained by these processes usually contain sulfur contaminants, such as hydrogen sulfide. When using gasification products as fuel, due to their 10 to 35 volume percent carbon monoxide, it is desirable to shift the products to enhance the hydrogen content of the fuel within the fuel cell. The initial carbon monoxide to hydrogen water-gas-shift catalytic activity of a conventional nickel molten carbonate fuel cell anode or of an added nickel catalyst for production of additional hydrogen is quickly poisoned by the presence of sulfur containing chemicals. Molten carbonate fuel cell performance losses are known to be caused by contamination of the porous nickel anode by sulfides which may be present in impure fuel gases, such as may be obtained from gasification of naturally occurring organic carbonaceous materials, such as coal. See, for example, "Effects of H.sub.2 S on Molten Carbonate Fuel Cells", Robert J. Remick, Process Report, U.S. Department of Energy Contract DE-AC21-83MC20212; DOE/MC/20212-2039 (DE 86010431) May 1986. Smith, S. W., Kunz, H. R., Vogel, W. M. and Szymanski, S. J., "Effects of Sulfur on Molten Carbonate Fuel Cells", (paper presented at Electrochemical Society Meeting, Montreal, Canada, May 1982), have reported complete failure of cell voltage in a molten carbonate fuel cell with a nickel anode when 2 ppm hydrogen sulfide, on a volume basis, was present in the fuel gas. The reduction in molten carbonate fuel cell performance to unsatisfactory levels with the presence of very small amounts of hydrogen sulfide in the fuel gas has been reported by several investigators: United Technologies Corporation, "Development of Molten Carbonate Fuel Cell Power Plant Technology", DOE/ET/15440-8 Quarterly Technical Progress Report No. 8, prepared for Department of Energy, under contract No. DE-AC01-79ET15440, February 1983; Vogel, W. M. and Smith, S. W., "The Effect of Sulfur on the Anodic H.sub.2 (Ni) Electrode in Fused Li.sub.2 CO.sub.3 -K.sub.2 CO.sub.3 at 650.degree. C.", J. Electrochem. Soc, 129 (7) 1441-45 (1982); Tang, T. E., Claar, T. D., and Marianowski, L. G., "Effects of Sulfur-Containing Gases on the Performance of Molten Carbonate Fuel Cells", Interim Report EM-1699 prepared for Electric Power Research Institute by Institute of Gas Technology, February 1981; Sammells, A. F., Nicholson, S. B., and Ang, P.G.P., "Development of Sulfur-Tolerant Components for the Molten Carbonate Fuel Cells", J. Electrochem. Soc. 127, 350 (1980); and Claar, T. D., Marianowski, L. G., and Sammells, A. F., "Development of Sulfur-Tolerant Components for Second-Generation Molten Carbonate Fuel Cells", Interim Report EM-1114, prepared for the Electric Power Research Institute by Institute of Gas Technology, July 1969. A review of the effect of sulfur containing compounds on molten carbonate fuel cells is given in Marianowski, L. G., "An Update of the Sulfur Tolerance of Molten Carbonate Fuel Cells", paper presented at Third Annual Contaminant Control in Hot Coal Derived Gas Streams, Washington, Pa., May 1983. The expense of using pure hydrogen gas fuels is high as is the expense of sufficient removal of sulfur containing contaminants to a level which maintains satisfactory fuel cell operation with many conventional anode materials.
U.S. Pat. No. 3,431,146 teaches a molten carbonate fuel cell having a fuel electrode of nickel, cobalt, or iron, exhibits at least a temporary increase in power output by addition of hydrogen sulfide to a hydrogen fuel gas. In specific examples, this patent teaches 2.0 volume percent hydrogen sulfide added to hydrogen fuel gas increased power output by 50 percent during a one minute flow of the added hydrogen sulfide.
U.S. Pat. No. 4,404,267 teaches an anode composite for molten carbonate fuel cells wherein copper or nickel/copper alloy plated ceramic particles form a porous anode composite with a bubble pressure barrier adjacent the electrolyte tile, the pores being sized significantly smaller than the pores of the anode and sized to fill with electrolyte. U.S. Pat. No. 4,448,857 teaches a similar plated ceramic particle porous structure suitable for a cathode composite and U.S. Pat. Nos. 4,423,122 and 4,386,960 teach a similar plated ceramic particle porous structure for an electrode for a molten carbonate fuel cell and U.S. Pat. No. 4,361,631 teaches a method for production of such electrodes. U.S. Pat. No. 4,507,262 teaches a porous sintered copper blank plate affixed to the face of a porous anode wherein the pores are filled with metal oxide by use of an organometallic precursor to provide a bubble pressure barrier. U.S. Pat. No. 3,970,474 teaches electrochemical generation of power from carbonaceous fuels in a cell having a porous cathode of ceramic material coated with copper oxide. Copper oxide oxygen electrodes have been used in fuel cells as taught by U.S. Pat. No. 2,830,109.